If people want to burn fat, detoxify livers, shrink prostates, avoid colds, stimulate brains, boost energy, reduce stress, enhance immunity, prevent cancer, extend lives, enliven sex or eliminate pain, all they have to do is walk in to a vitamin store and look around.

The shelves will be lined with ginkgo or rose and orange oils touted as aids for memory; guarana and cordyceps for energy; chicory root for constipation; lemon balm oil, ashwagandha, eleuthero, Siberian ginseng and holy basil for stress; sage and black cohosh for menstrual pain; coconut oil and curry powder for Alzheimer’s disease; saw palmetto for prostate health; sandalwood bark to prevent aging; garlic for high cholesterol; peppermint oil for allergies; artichoke extract and green papaya for digestion; echinacea for colds; chondroitin sulfate and glucosamine for joint pain; milk thistle for hepatitis; St. John’s wort for depression; and tongkat ali for sexual potency.

The question, however, is: Which products work? And how do we know they work? Fortunately, thanks to James Lind, we can figure it out.

When Lind climbed aboard the HMS Salisbury intent on testing whether citrus was a cure for scurvy in 1740, he moved medicine from a faith-based system to an evidence-based system. No longer do we believe in treatments. We can test them to see whether they work.

Although the size and cost of clinical studies have increased dramatically since the days of Lind, the claims made about alternative remedies are testable, eminently testable.

In that sense, there’s no such thing as alternative medicine. If clinical trials show that a therapy works, it’s good medicine. And if a therapy doesn’t work, then it’s not an alternative.

For example, Hippocrates used the leaves of the willow plant to treat headaches and muscle pains. By the early 1800s, scientists had isolated the active ingredient: aspirin. In the 1600s, a Spanish physician found that the bark of the cinchona tree treated malaria. Later, cinchona bark was shown to contain quinine, a medicine now proven to kill the parasite that causes malaria. In the late 1700s, William Withering used the foxglove plant to treat people with heart failure. Later, foxglove was found to contain digitalis, a drug that increases heart contractility. More recently, artemisia, an herb used by Chinese healers for more than a thousand years, was found to contain another anti-malaria drug, which was later called artemisinin.

“Herbal remedies are not really alternative,” writes Steven Novella, a Yale neurologist. “They have been part of scientific medicine for decades, if not centuries. Herbs are drugs, and they can be studied as drugs.”

Looking at the claims

In many case, though, when natural products have been put to the test, they’ve fallen short of their claims. For instance, although mainstream medicine hasn’t found a way to treat dementia or enhance memory, practitioners of alternative medicine claim that they have: ginkgo biloba. As a consequence, ginkgo is one of the 10 most commonly used natural products.

Yet between 2000 and 2008, the National Institutes of Health funded a collaborative study by the University of Washington, the University of Pittsburgh, Wake Forest University, Johns Hopkins University and the University of California at Davis to determine whether ginkgo worked. More than 3,000 elderly adults were randomly assigned to receive ginkgo or a placebo. Decline in memory and onset of dementia were the same in both groups. In 2012, a study of more than 2,800 adults found that ginkgo didn’t ward off Alzheimer’s disease.

Another example is St. John’s wort. Every year, 10 million people suffer major depression in the United States, and every year 35,000 people kill themselves. Depression is a serious illness; to treat it, scientists have developed medicines that alter brain chemicals such as serotonin. Called selective serotonin reuptake inhibitors (SSRIs), these drugs are licensed by the FDA and have been shown to help with severe depression.

But some people have heard there’s a more natural, safer way to treat severe depression: St. John’s wort. Because so many people use the herb and because depression, if not properly treated, can lead to suicide, researchers studied it. Between November 1998 and January 2000, 11 academic medical centers randomly assigned 200 outpatients to receive St. John’s wort or a placebo: The results showed no difference in any measure of depression.

Another favorite home remedy is garlic, to lower cholesterol. Because high cholesterol is associated with heart disease, because heart disease is a leading cause of death, because lipid-lowering agents lower cholesterol and because many people are choosing garlic instead of lipid-lowering agents, researchers studied it. In 2007, Christopher Gardner and co-workers at Stanford University School of Medicine evaluated the effects of garlic on 192 adults with high levels of low-density lipoprotein cholesterol (bad cholesterol). Six days a week for six months, participants received either raw garlic, powdered garlic, aged garlic extract or a placebo. After checking cholesterol levels monthly, investigators concluded, “None of the forms of garlic used in this study . . . had statistically or clinically significant effects on low-density lipoprotein cholesterol or other plasma lipid concentrations in adults with moderate hypercholesterolemia.”

Saw palmetto for the prostate is also popular. As a man ages, his prostate enlarges, which blocks the flow of urine. If untreated, prostate enlargement can cause urinary tract infections, bladder stones and kidney failure. Medicines that relax muscles within the prostate or reduce its size have been available for years. But more than 2 million men turn to saw palmetto instead.

In 2006, the National Center for Complementary and Alternative Medicine, part of the National Institutes of Health, supported a study at the University of California at San Francisco, the San Francisco Veterans Affairs Medical Center and Northern California Kaiser Permanente. Investigators assigned 225 men with moderate to severe symptoms of prostate enlargement to receive either saw palmetto or a placebo twice daily for a year: They found no difference between the two groups in urinary flow rate, prostate size or quality of life.

Five years later, the study was repeated with 369 men, this time with higher doses. Again, no change in urinary symptoms. “Now we know that even very high doses of saw palmetto make absolutely no difference,” said study author Gerald Andriole. “It clearly does not work any better than a sugar pill.”

Another popular remedy is milk thistle, which some have said can help patients with chronic hepatitis or other liver problems.

In 2011, Michael Fried of the University of North Carolina at Chapel Hill led a group of investigators testing those claims. More than 150 people infected with hepatitis C virus were given either milk thistle or a placebo. Then investigators determined the amount of liver damage, as well as the quantities of hepatitis C virus in blood. They found no difference between the two groups.

And what about chondroitin sulfate and glucosamine for joint pain? In 2006, Daniel Clegg of the University of Utah led a group of investigators to see whether it worked. They studied more than 1,500 people with knee osteoarthritis who were given either chondroitin sulfate alone, glucosamine alone, both, a placebo or Celebrex (an FDA-licensed anti-inflammatory drug). Only Celebrex worked for all subgroups in the study.

One of the most popular herbal remedies in the United States is echinacea. Used to treat colds, it’s a $130-million-a-year business. In 2003, James Taylor and co-workers at the University of Washington in Seattle studied more than 400 children with colds who had received either echinacea or a placebo for 10 days. The only difference: Children taking echinacea were more likely to develop a rash.

Some do work

Yet, some dietary supplements might be of benefit for otherwise healthy people: calcium and Vitamin D in post-menopausal women to prevent bone thinning, and folic acid during pregnancy to prevent birth defects.

Calcium is the most abundant mineral in the body, required for vascular tone, muscle function, nerve transmission and hormone secretion. Less than 1 percent of total body calcium is necessary for performing these functions. The remaining 99 percent is stored in bones, where it supports bone structure and function. The problem with calcium occurs when people get older.

In children and teenagers, bone formation exceeds bone breakdown. In early and middle adulthood, these two processes occur at equal rates. Past the age of 50, however, especially in post-menopausal women, bone destruction exceeds bone formation, leaving them vulnerable to fractures. Every year, more than 1.5 million fractures occur in the United States because of bone thinning. The best way to avoid this problem is to eat dairy products such as milk, yogurt and cheese, or calcium-fortified fruit juices, cereals and other foods.

Because most women get enough calcium in their diet and because supplementary calcium has not been shown to reduce fractures in otherwise healthy post-menopausal women, the U.S. Preventive Services Task Force does not recommend supplemental calcium.

But calcium is linked to Vitamin D. People who take in adequate amounts of calcium might still have a problem with bone strength if they do not also get enough Vitamin D, which helps the body absorb calcium. To get an adequate amount of Vitamin D, people need only expose their face, arms, hands or back to sunlight (without sunblock) for 10 to 15 minutes a day at least twice a week. This will provide the 600 international units of Vitamin D recommended by the Institute of Medicine.

Some people, however, can’t or don’t do this. For this reason, many foods are supplemented with Vitamin D, such as milk, bread, pastries, oil spreads, breakfast cereals and some brands of orange juice, yogurt, margarine and soy beverages. Because most people get enough Vitamin D in their foods or from exposure to sunlight, the U.S. Preventive Services Task Force does not recommend supplemental Vitamin D.

There are, however, two exceptions: Babies who are exclusively breast-fed should receive 400 international units a day of supplemental Vitamin D, because it isn’t contained in human milk and because they don’t get out into the sun much; and adults older than 65 should receive 800 units daily because this has been shown to reduce the high risk of bone fractures.

Finally, folic acid is a B-complex vitamin necessary for the production of red blood cells. Without folic acid, people develop anemia. Researchers have shown that folic-acid deficiency can also cause something far worse: severe birth defects. Pregnant women deficient in folic acid have delivered babies with malformations of the spine, skull and brain. To avoid folic-acid deficiency, people need about 400 micrograms a day.

Foods rich in folic acid include vegetables such as spinach, broccoli, lettuce, turnip greens, okra and asparagus; fruits such as bananas, melons and lemons; and beans, yeast, mushrooms, beef liver and kidney, orange juice and tomato juice. Although there are plenty of sources of this nutrient, many pregnant women weren’t getting enough folic acid in their diets. So in 1998, the FDA required manufacturers to add folic acid to breads, breakfast cereals, flours, cornmeals, pastas, white rice, bakery items, cookies, crackers and some grains. As a result, it is now almost impossible to become folic-acid deficient.

Nonetheless, women of childbearing years are advised to take 400 micrograms of folic acid every day, obtained either from foods or supplements or both.

In the end, if a medicine works (such as folic acid to prevent birth defects), it’s valuable; and if it doesn’t work (such as saw palmetto to shrink prostates), it’s not. “There’s a name for alternative medicines that work,” says Joe Schwarcz, professor of chemistry and the director of the Office for Science and Society at McGill University in Montreal. “It’s called medicine.”

Offit is chief of the division of infectious diseases and director of the Vaccine Education Center at the Children’s Hospital of Philadelphia. This article was excerpted from his new book, “Do You Believe in Magic? The Sense and Nonsense of Alternative Medicine,” copyright 2013 by Paul Offit, M.D. Reprinted by permission of Harper, an imprint of HarperCollins Publishers.